2 Properties of Aqueous Solutions of Acids and Bases acidic solutions They have a sour taste They change the colors of many indicators React with metals to generate hydrogen They react with metal oxides and hydroxides to form salts and water Acidic aqueous solutions conduct electricity basic solutions They have a bitter taste They change the colors of many indicators They react with acids to form salts and water Aqueous basic solutions conduct electricity 2

4 The Arrhenius Theory Svante Augustus Arrhenius first presented this theory of acids and bases in Acids are substances that contain hydrogen and produces H + in aqueous solutions. Bases are substances that contain the hydroxyl, OH, group and produce hydroxide ions, OH -, in aqueous solutions. 4

5 The BrØnsted-Lowry Theory J.N. BrØnsted and T.M. Lowry developed the acid-base theory in An acid is a proton donor (H + ). A base is a proton acceptor. 5

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10 The BrØnsted-Lowry Theory An important part of BrØnsted-Lowry acid-base theory is the idea of conjugate acid-base pairs. Two species that differ by a proton are called acid-base conjugate pairs. HNO 3 + H 2 O H 3 O + + NO 3 - HNO 3 is the acid, conjugate base is NO 3 - H 2 O is the base, conjugate acid is H 3 O + 10

11 The BrØnsted-Lowry Theory Conjugate acid-base pairs are species that differ by a proton. 11

12 The BrØnsted-Lowry Theory The major differences between Arrhenius and Brønsted-Lowry theories. 1. The reaction does not have to occur in an aqueous solution. 2. Bases are not required to be hydroxides. 12

13 The BrØnsted-Lowry Theory An important concept in BrØnsted-Lowry theory involves the relative strengths of acid-base pairs. Weak acids have strong conjugate bases. Weak bases have strong conjugate acids. The weaker the acid or base, the stronger the conjugate partner. The reason why a weak acid or base is weak is because the conjugate is so strong it reforms the original acid or base. 13

14 The BrØnsted-Lowry Theory NH + H O NH + OH Since NH 3 is a weak base, NH 4+ must be a strong acid. NH 4+ gives up H + to reform NH Compare that to NaOH Na + (aq) + OH - (aq) Na + must be a weak acid or it would recombine to form NaOH 14

15 The Lewis Theory Developed in 1923 by G.N. Lewis. Emphasis on what the electrons are doing as opposed to what the protons are doing. Acids are defined as electron pair acceptors. Bases are defined as electron pair donors. 15

16 The Lewis Theory One Lewis acid-base example is the ionization of ammonia. Look at this reaction in more detail paying attention to the electrons. H + H N H H + H O H H N H H + O H - Base - it donates the electron pair Acid - it accepts the electron pair Notice that a coordinate covalent bond is formed on the ammonium ion. 16

24 The Autoionization of Water Water can be either an acid or base in Bronsted-Lowry theory. Consequently, water can react with itself. This reaction is called autoionization. One water molecule acts as a base and the other as an acid. H O H O + + H O + 2 base 1 2 acid 2 3 acid 1 OH base

25 The Hydronium Ion (Hydrated Hydrogen Ion) The protons that are generated in acid-base reactions are not present in solution by themselves. Protons are surrounded by several water molecules and called the hydronium ions. H + (aq) is really H(H 2 O) n + 25

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28 Amphoterism Species that can behave as an acid or base are called amphoteric. 28

34 Strengths of Acids Ternary acid strength usually increases with: 1. an increasing number of O atoms on the central atom. 2. an increasing oxidation state of central atom. Every additional O atom increases the oxidation state of the central atom by 2. 34

35 Strengths of Acids For ternary acids having the same central atom: the highest oxidation state of the central atom is usually strongest acid. For example, look at the strength of the Cl ternary acids. HClO < HClO 2 < HClO 3 < HClO 4 weakest strongest 35

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